Together with diet and behavioural modification, regular exercise is one of the key components of programs for the treatment of obesity. it appears to be one of the major factors determining the rang-term success of weight loss programs. One of the mechanisms that may underlie this and other beneficial effects of regular exercise is the effect of exercise training on substrate utilisation. Exercise training increases fat oxidation in lean subjects. The capacity to mobilise and oxidise fat has been shown to be impaired in obese and post-obese subjects. Thus, an increase in the capacity for fat oxidation may help to maintain fat and energy balance at a lower fat mass in individuals with a predisposition for obesity. However, in view of the impaired fat oxidation in obese and post-obese individuals the question arises whether exercise training also increases fat oxidation in the obese. Few studies have addressed this question and have investigated the effect of exercise training on substrate utilisation in obesity. Addition of an exercise training program has been shown to prevent the reduction of basal fat oxidation that is associated with diet-induced weight loss in two studies. No effect of additional exercise training on substrate oxidation during exercise was found. In post-obese subjects exercise training caused no change in 24 h substrate oxidation in one study and increased carbohydrate, rather than fat oxidation in another. In obese subjects neither low (40% maximal aerobic fitness (VO(2)max)) nor high (70% VO(2)max) intensity training was found to affect resting substrate oxidation. During exercise fractional fat oxidation was increased after low, but not after high intensity training. The question, whether exercise training increases fat oxidation in obese and post-obese as in lean subjects therefore cannot be answered conclusively at this point in time and requires further study. The role of exercise intensity and type of exercise needs to be studied as well.